1. Existing Need
Electrically rechargeable batteries are strategic components of electric vehicles. The goal is to have high energy density, high power density, be environmentally friendly, cheap and safe batteries. At the moment Lead-acid, NiMH and different types of Li-ion batteries are favorite for EV usage but they all suffer at least one drawback, mainly cost or safety in case of high energy density battery.
Metal-air cells are well-known to be relatively light-weight power sources. They utilize oxygen from ambient air or dissolved into water as a reactant in an electrochemical reaction.
Metal-air batteries comprise a negative metal electrode (e.g., zinc, aluminum, magnesium, iron, lithium, etc.) and a positive electrode having a porous structure with catalytic properties for an oxygen reduction reaction (typically referred to as the air-electrode of the battery). An electrolyte is used to maintain high ionic conductivity between the two electrodes. For alkaline metal-air batteries (i.e., having an alkaline electrolyte), the air electrode is usually made from thin, porous polymeric material (e.g., polytetrafluoroethylene) bonded carbon layers. To prevent a short circuit of the battery, a separator is provided between the negative electrode (anode) and the positive electrode (cathode).
In the present description what is called “air-electrode” includes both cases: (i) gas-diffusion electrode consuming oxygen from a gas containing oxygen (typically air) or (ii) gas-diffusion electrode consuming oxygen from a liquid containing dissolved oxygen gas (typically seawater); and what we call “electrolyte side” of the air-electrode is the side to be bathed by the electrolyte.
Also the electrolyte is most often alkaline but it could also be acidic, neutral (salt electrolyte) or aprotonic (non-aqueous).
During discharging of metal-air batteries, in alkaline media, oxygen is converted to hydroxide ions in the air-electrode. The reaction in the air-electrode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxide ions. The hydroxide ions migrate and diffuse through the electrolyte toward the metal negative electrode, where oxidation of the metal of the negative electrode occurs, forming oxides or hydroxides or ions and liberating electrons. In a secondary (i.e. rechargeable) metal-air battery, charging is induced by applying external voltage between the metal-electrode and the air-electrode or an auxiliary-electrode to reverse the electrochemical reactions. Charging converts hydroxide ions to oxygen on the air-electrode or the auxiliary-electrode, releasing electrons, while, at the metal-electrode, the metal oxides or ions are reduced back to form the metal while electrons are consumed.
Metal-air batteries could offer electrically rechargeable batteries with excellent characteristics, but so far no bifunctional air-electrode was available that would allow a metal-air battery to be recharged at acceptable rates.
2. Description of the Prior Art
Some tried to counter the problem in recharging with the help of an auxiliary electrode, for example Ni-metal or Ni-oxide electrode, placed between the metal and the air electrode or placed at the other side of the metal electrode. But this solution is leading to lower characteristics of the battery cell, typically the auxiliary electrode is adding some weight and volume and increasing internal resistance in case it is on the side of the air-electrode, thus the global specific energy density in Wh/kg or Wh/l is lower than cells not employing auxiliary electrode. Also, charging and discharging schemes for a cell with auxiliary electrode is somehow complicated vs. conventional two-lead bifunctional cells as batteries with auxiliary electrode have three leads. Especially if bifunctional air-electrodes are available then a metal electrode can be placed between 2 bifunctional air-electrodes. This cell setup, which is conventional, leads to increased specific power and energy density, plus ease of operation vs. the three-lead auxiliary-electrode cell setup.
A major problem of classical bifunctional air-electrodes is that:
material with catalytic activity, such has carbon black or some specific catalyst, is present in the active layer of the air-electrode in order to reduce oxygen during discharge and to provide means for the air-electrode to consume electrons;
but unfortunately, during recharging, when the air-electrode is under anodic polarization, the said material with catalytic activity along with the active layer material itself are degenerated, the air-electrode losing most of its efficiency and becoming not usable any longer. Indeed a classical issue is the oxidation of carbon black with coloration of electrolyte, which is mainly due to carbon black releasing hydroquinone into the electrolyte when being under the anodic polarization, this leading to loss of its activity.
Therefore there is a need for bifunctional air-electrodes and for secondary metal-air batteries that do not prematurely fail due to deterioration during recharging and that support a large number of discharge/recharge cycles.